Typical examples of fluorescent semiconductor nanoparticles currently known are nanoparticles of II-VI semiconductors and III-V semiconductors, which have a diameter of about 1.5 to about 15 nm. These semiconductor nanoparticles are produced by the solution method, and have been actively researched as a new type of fluorescent material. These particles have so-called “quantum size effects”. That is, even if made of the same material, the smaller particles have a wider band gap, and thus emit shorter wavelengths of light.
Such nanoparticles have the following three notable characteristics:    1. Due to their high proportion of surface atoms, the nanoparticles are prone to aggregate, unless covered appropriately.    2. Because the nanoparticles have many defects on their surface, a high PL efficiency cannot be obtained unless the defects are carefully removed.    3. Water-dispersibility is very important to cover the nanoparticles with glass by a sol-gel process, or to use the nanoparticles as a biological fluorescent probe.
To produce a material more advantageous for application, based on the understanding of these three characteristics, the surface design of the nanoparticles is very important.
An examination of the history of semiconductor nanoparticle research shows that most of the research studies conducted thus far have concerned II-VI semiconductors. CdS nanoparticles in a glass were reported as early as 1926 (Non-Patent Document 1). Later, around 1982, CdS nanoparticles were produced by a reaction in a colloidal solution, and discussions began regarding the physical properties thereof, such as luminescence (Non-Patent Document 2).
Thereafter, for a certain period of time, German research groups conducted a number of research studies. Further, in first half of the 1990's, many attempts were made to produce nanoparticles in glass, not in a colloidal solution (Non-Patent Documents 3 and 4). Later, around 1997, two American research groups produced CdSe nanoparticles with a narrow particle size distribution and a high PL efficiency by a method comprising thermally decomposing an organometallic compound in an organic solvent under a substantially water- and oxygen-free atmosphere. This brought a great deal of attention to research in this field (Non-Patent Documents 5 and 6).
It is generally known that covering nanoparticles with another semiconductor material having a large band gap remarkably increases the PL efficiency. ZnSe or ZnS is preferably used as a semiconductor layer for covering CdSe. The inner semiconductor (CdSe in the above example) is called a “core”, whereas a semiconductor for coating the inner semiconductor (ZnSe or ZnS in the above example) is called a “shell”; and the entire structure such as a CdSe/ZnS nanoparticle structure is called a “core/shell structure”.
The present inventors devoted their attention to a simple and easy method using water as a solvent in place of an organic solvent, and succeeded in synthesizing CdTe nanoparticles with a high PL efficiency (Non-Patent Document 7, Patent Document 1). In this method, CdS is a shell and covered on its outer surface with thioglycolic acid (TGA) to provide high water dispersibility.
Because Cd has a high toxicity, research was also conducted for the replacement of Cd with Zn. A method comprising irradiation with UV light in an aqueous solution containing a surfactant and zinc ion was reported to be effective to obtain high PL efficiency and water dispersibility (Non-Patent Document 8, Patent Document 2).
However, such II-VI semiconductors have strong ionic bonding character, and therefore have unsatisfactory durability. Furthermore, semiconductors with a high PL efficiency have a defect in that they contain toxic metals, such as Cd and Hg. In contrast, III-V semiconductors have strong covalent bonding character and are therefore satisfactorily durable. However, compared to II-VI semiconductors, it is difficult to produce III-V semiconductors; high temperature and high pressure conditions are required. For this reason, much less research has been conducted on III-V semiconductors than on II-VI semiconductors. However, several attempts have been made in the last 6 to 7 years.
First, it was found that InP nanoparticles treated with hydrofluoric acid emit light (Non-Patent Document 9). To achieve a high PL efficiency with high reproducibility, a method comprising treatment with hydrofluoric acid while applying light was developed as an improvement of this method (Non-Patent Document 10). In a similar year, it was also found that covering the surface of InP with ZnS, as in the case of II-VI, also increases the PL efficiency (Non-Patent Document 11). Attempts were also made to cover InAs with various shell materials (Non-Patent Document 12). According to the research studies conducted thus far, III-V nanoparticles were dispersed in an organic solvent, and the PL efficiency was about 50% at most. When treating nanoparticles with hydrofluoric acid, water containing hydrogen fluoride was usually added in an amount of about 10 volume % to an organic solvent, and water-dispersible III-V nanoparticles were not obtained at this level of prior art method.
Recently, water-dispersible nanoparticles were produced by bonding TGA molecules to the outer surface of InP/ZnS core/shell structured nanoparticles (Non-Patent Document 13). In this method, the PL efficiency of nanoparticles before bonding of TGA molecules thereto was reportedly 15%. The PL efficiency of nanoparticles to which TGA molecules have been bonded is thus lower than that value. Around the same period, it was also reported that the InAsxP1-x/InP/ZnSe core/shell structure (wherein InP and ZnSe are the shell) covered with phospholipid can be dispersed in water, and has a PL efficiency of 3.5% (Non-Patent Document 14, Patent Document 3). Later, the same research group also reported that water-dispersible nanoparticles with a PL efficiency of 6 to 9% can be produced by using InAs nanoparticles with a diameter of 2 nm or less as a core (Non-Patent Document 15). Further, in recent years, it was reported that InP/ZnS nanoparticles produced by an improved shell production method achieves a PL efficiency of 40%, and can be transferred to water (Non-Patent Document 16). In this method, however, a high reaction temperature, i.e., about 220° C., is required.
Below are summaries of earlier patents. Patent Document 4 discloses water-dispersible, light-emitting III-V nanoparticles. Patent Document 5 states that a nanoparticle complex that can be stably maintained with a high PL efficiency in water can be obtained by forming a metal layer on the surface of III-V nanoparticles. However, these patent documents do not specifically disclose a method for obtaining a high PL efficiency; and there is no specific description about stability.
As described above, known methods of producing III-V semiconductor nanoparticles require a high temperature, i.e., 200° C. or more, and fail to disperse nanoparticles in water while high PL efficiency is maintained. Therefore, highly stable III-V nanoparticles that can be stably dispersed in water while maintaining the PL efficiency would be very advantageous for application.
Regarding the compositional ratio of the III-V nanoparticles, the Group III element/Group V element ratio has been continuously examined since the early days of nanoparticle production. An early report showed that the In/P molar ratio is in the range of 1 to 1.1 (Non-Patent Document 17). A subsequent report showed that the In/P ratio is in the range of 1 to 1.1 (Non-Patent Document 18). A later report showed In/P=1.2 (Non-Patent Document 19). These reports indicate that the molar ratio of the Group III element to the Group V element is in the range of 1 to 1.2.